Runcorn was educated at the University of Cambridge (B.A., 1944; M.A., 1948) and the University of Manchester (Ph.D., 1949). He was assistant director of geophysics research at Cambridge from 1950 to 1955, and from 1956 to 1988 he was professor of physics and head of the physics department at King’s College, which was part of the University of Durham and became the University of Newcastle upon Tyne in 1963.

In the late 1940s and ’50s Runcorn helped establish the field of paleomagnetism—i.e., the study of the residual magnetization that is evident in ancient rocks. Such rocks preserve fossilized traces of the direction of the Earth’s magnetic field that prevailed at the time the rocks were formed. Runcorn’s analyses of rocks in Europe provided evidence of periodic reversals of the Earth’s field (geomagnetic polar reversals) over geologic time. Moreover, his data suggested that the Earth’s north magnetic pole had moved, or wandered, widely over hundreds of millions of years. Runcorn’s first explanation was that the geographic pole of the Earth had itself migrated, but this was contradicted by evidence that the drift of the magnetic pole as shown by American rocks was different from that shown by European ones. The magnetic curves of the European and American rocks could be aligned, or reconciled, however, on the assumption that those two continents had formerly been joined and had subsequently drifted apart into their present-day positions. Impressed by this result, Runcorn became an early proponent of the theory of continental drift, and the paleomagnetic data obtained by him and other researchers eventually provided some of the strongest evidence in support of the theory.